Field
The present invention relates to microelectromechanical devices and to a gyroscope structure and a gyroscope.
Description of the Related Art
Micro-Electro-Mechanical Systems, or MEMS, can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. Since MEMS devices are created with the same tools used to create integrated circuits, micromachines and microelectronics can be fabricated on the same piece of silicon or other substrate to enable advanced machines.
MEMS structures can be applied to quickly and accurately detect very small changes in physical properties. For example, a microelectromechanical gyroscope can be applied to quickly and accurately detect very small angular displacements. Motion has six degrees of freedom: translations in three orthogonal directions and rotations around three orthogonal axes. The latter three may be measured by an angular rate sensor, also known as a gyroscope. MEMS gyroscopes use the Coriolis Effect to measure the angular rate. When a mass is moving in one direction and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. The resulting physical displacement caused by the Coriolis force may then be read from, for example, a capacitive, piezoelectrical or piezoresistive sensing structure.
In MEMS gyros the primary motion cannot be continuous rotation as in conventional ones due to lack of adequate bearings. Instead, mechanical oscillation may be used as the primary motion. When an oscillating gyroscope is subjected to an angular motion orthogonal to the direction of the primary motion, an undulating Coriolis force results. This creates a secondary oscillation orthogonal to the primary motion and to the axis of the angular motion, and at the frequency of the primary oscillation. The amplitude of this coupled oscillation can be used as the measure of the angular rate.
Gyroscopes are very complex inertial MEMS sensors, and still the tendency is towards more and more compact structures. The basic challenge in gyroscope designs is that the Coriolis force is very small and therefore the generated signals tend to be minuscule compared to other electrical signals present in the gyroscope. Spurious responses and susceptibility to vibration plague many compact MEMS gyro designs, like conventional tuning fork structures.
One known approach to reduce sensitivity to external vibrations is a balanced ring structure that includes a planar vibratory resonator that has a ring or hoop like structure with inner or outer peripheries extending around a common axis. The planar resonators are typically excited into a cos 2θ resonance mode that exists as a degenerate pair of vibration modes at a mutual angle of 45°. One of these modes is excited as the carrier mode. When the structure is rotated around the axis normal to the plane of the ring, Coriolis forces couple energy into a response mode. The amplitude of motion of the response mode gives a direct measure of the applied rotation rate.
A disadvantage of the ring structure is that the sectional seismic masses and their displacements are relatively small, resulting to low signal levels. Also the dimensions of the sectional actuation and sensing structures make the functions indistinguishable and thus compromised.